Active and passive boundary layer control for vehicle drag reduction

ABSTRACT

An active and passive boundary layer control system and method for reducing boundary layer separation and the resulting form drag from ground vehicles. Tubings with multiple venturis connected in series are attached to the side and/or roof of the ground vehicle. A flow of compressed air is created by an engine auxiliary, e.g., a turbocharger or a supercharger, or by a designated blower and injected into the tubings. Suction created by the venturis actively removes decelerated fluid from the boundary layer and keeps the boundary layer flow attached. Optionally, the system exhausts the air in concentrated free jets along the vehicle body to a series of passive boundary layer control devices, such as vortex generators.

FIELD OF THE DISCLOSURE

This disclosure relates to a device and a method for reducing vehicledrag of a ground vehicle traveling at speed. Active and/or passiveboundary layer control elements for preventing boundary layer separationare attached to a side wall and/or the roof of the ground vehicle.

BACKGROUND

Aerodynamic drag is created whenever a ground vehicle travels at speed.Aerodynamic drag is comprised of two principle components—skin frictiondrag and form drag. The skin friction drag is a consequence of the air'sviscosity and the “no-slip” condition that exists at the vehicle'ssurface. Even for the flow of fluids of very low viscosity—air forexample—there is a region where the effects of the fluid's viscositydominates. This area is called the boundary layer. FIG. 1 shows the flowof a freestream of air 1, over a stationary wall 2. As a result of theno-slip condition 3 at the wall, the velocity at the wall 2 is zero. Thelayer of fluid that corresponds to the distance from the no-slipcondition 3 at the wall to the re-establishment of the freestreamvelocity is called the boundary layer 4.

Within the boundary layer, adjacent layers of fluid will be traveling atdifferent velocities. The different velocities are the result ofshearing stresses that are produced in the fluid. The shearing stressesare produced by the fluid's viscosity. Outside the boundary layer—in thefreestream—all fluid will be traveling at the same speed and the effectof the fluid's viscosity will be negligible. For the specific case of aground vehicle, the no-slip condition dictates that the air immediatelyadjacent to the vehicle will travel at the same speed as the vehicle. Asdiscussed above, outside the boundary layer the air will be traveling atthe air's free stream velocity. For the purposes of this discussion itwill be assumed that the vehicle is stationary and the air is moving atthe vehicle's velocity. This convention does not change the physicsinvolved but it does make for an easier situation to describe. Theshearing stresses in the boundary layer that produce the velocitygradient across the boundary layer also act as a retarding force on thevehicle's motion—i.e., drag. This component of drag is called skinfriction drag.

For high speed, streamlined vehicles like jet aircraft, skin frictiondrag is the major contributor to aerodynamic drag. FIG. 2 shows the flowof air over a symmetric airfoil, 5, with chord, 6. A freestream flow ofair, 1, is introduced and directed over the airfoil, 5. Because of thestreamlined shape of the airfoil, 5—only gradual changes in surfaceprofile exist—the flow is largely able to proceed in a direction ofdecreasing pressure. As a result the boundary layer flow remains“attached” to the airfoil, 5. The attached flow is shown as 7.

However, for ground vehicles—particularly for bluff, i.e.,non-streamlined bodies like tractor trailer trucks—the major contributorto drag is form drag. FIG. 3 shows the flow of a freestream of air overa typical automobile 8. Form drag is created by separation of theboundary layer from the vehicle. As described above, the shearingstresses present in the boundary layer cause the air in the boundarylayer to slow down relative to the speed of the vehicle. As a result ofits reduced energy content the boundary layer of air may no longerremain attached to the vehicle, particularly when the air is forced toflow into an area of increasing pressure. Areas of increasing pressureare produced by surfaces with a large radius of curvature or a geometricdiscontinuity. Areas of increasing pressure can be reduced by vehiclestreamlining but there are practical limits on what can be achieved withstreamlining commercial vehicles. In the flow of air around conventionalground vehicles areas of increasing pressure cannot be avoided. Inplaces of increasing pressure the air can begin to recirculate or flowin the opposite direction.

This flow in areas of increasing pressure is seen in FIG. 3 as the airtries to flow along the vehicle's rear deck 9. The large radius ofcurvature of the deck leads to a large increase in area for the air toflow through and the air will slow down as a result. This is the samephenomenon that occurs in a diffuser and as with a diffuser the flowalong the rear deck 9 leads to an increase in pressure. If the increasein pressure is sufficiently large the boundary layer flow of air aroundthe vehicle can separate from the vehicle. Boundary layer separation isseen as 10. As the boundary layer separates, a large low-pressure wake11 is created behind the vehicle. Inside this wake the air is at a lowerpressure than it is at the vehicle front. The pressure gradient acrossthe vehicle produces a net force that acts to prevent movement of thevehicle—i.e., drag. This component of drag is called form drag.

The separation of the boundary layer flow of air over a ground vehicleis identical to the stall of an aircraft wing. When a wing is generatinglift, the flow of air around the wing remains attached to the wing. SeeFIG. 2. As long as the flow remains attached, the form drag produced bythe wing remains very small. FIG. 4 shows an airfoil 5 of chord 6. Thewing's angle of attack 12 is the angle the chord 6 forms with thefreestream 1. As the wing's angle of attack 12 is increased, a pointwill be reached where the boundary layer flow of air separates from thewing 10. The location where the flow separates is called the separationpoint 13. As a result of this separation a large wake 11 will beproduced behind the wing. The wake 11 causes the drag produced by thewing to rapidly increase. In addition, the separation of the flow alsoproduces a large reduction in lift generated by the wing. The reductionin lift is the result of large areas of the wing no longer being exposedto the boundary layer flow of air and thus being unable to produce lift.In order to avoid the consequences of boundary layer separation and wingstall, aircraft are routinely provided stall warning devices. Uponreceiving an alarm from the stall warning device, a pilot will typicallyreduce the aircraft's angle of attack by pitching the nose down orrolling the wings level.

Airplane wings are highly streamlined and operate at very high speeds,especially when compared to the speed at which a ground vehicletypically travels. Wings are designed to operate with very little formdrag because the boundary layer flow of air around the wing remainsattached under all operating scenarios. An example of form drag on anon-streamlined object for illustrating ground vehicle performance isprovided by a golf ball.

Most ground vehicles—particularly tractor trailer trucks—are bluffbodies, just like a golf ball. FIG. 5 shows a golf ball 14 exposed to afreestream flow of air 1. The separation of flow from the golf ball isseen as 13. As a result of this separation of flow, a large low pressurewake 11 is created behind the ball. The large pressure difference acrossthe golf ball opposes the motion of the golf ball and produces drag.Unlike the operation of an airplane wing where separation must beavoided at all cost—hence the use of stall warning systems and highlystreamlined shapes—it is a foregone conclusion that the flow of airaround a golf ball will result in boundary layer separation. This is theresult of the golf ball's non-streamlined shape. Once it is acceptedthat the boundary layer will separate from a golf ball in flight, thequestion becomes how to minimize the form drag resulting from the largelow pressure wake 11 created by the boundary layer separation. Theanswer is to add dimples to the golf ball.

There are two types of boundary layer flow—laminar and turbulent. Inlaminar boundary layers, the flow is well ordered and all layers of floware essentially parallel to each other. In contrast, turbulent flow ismuch more chaotic and unordered. There is also significant mixingbetween adjacent layers of flow in turbulent boundary layers. As aresult of this mixing and the higher energy levels that exist inturbulent boundary layers, turbulent boundary layers will remainattached longer than laminar boundary layers. The dimples on a golf ballare designed to “trip” the boundary layer flow from laminar toturbulent. By tripping the boundary layer into a turbulent flow regime,separation of the boundary layer from the golf ball is delayed and formdrag is reduced. The practical result of the reduced form drag is a golfball that travels farther. The effect of adding dimples to a golf ballcan be seen in FIG. 6, which shows a golf ball 14 again exposed to afreestream flow of air, 1. The golf ball contains dimples, 15. Thedimples trip the boundary layer from laminar to turbulent. Thisincreases the energy in the boundary layer and delays separation.Comparison of FIG. 6 to FIG. 5 shows that separation 13 now occurs laterwhen the ball is dimpled. The delayed separation reduces the size of thelow pressure wake 11 and allows the dimpled ball to travel further thanthe “non-dimpled” ball.

The distinction between skin friction drag resulting from the effects ofviscosity in the boundary layer and form drag resulting from boundarylayer separation and the creation of a low pressure wake along with thedifferent techniques that can be utilized to minimize form drag havebeen known for some time. Indeed, Ludwig Prandtl—who is credited withidentifying the boundary layer—studied the effect of suction on boundarylayer separation as far back as 1904. More recently, several patentcitations discuss active control of the boundary layer system in orderto reduce vehicle drag.

U.S. Pat. No. 4,146,202 pertains to a porous aircraft skin that can beused with a suction source to maintain laminar flow over an aircraftwing. A system that utilized both suction and cooling via a cryogenicfluid to reduce aircraft drag is disclosed in U.S. Pat. No. 4,807,831.The cryogenic fluid can be provided from the aircraft's propulsionsystem—liquid hydrogen and liquid oxygen. As a result, it is notnecessary to carry a cryogenic fluid for the sole purpose of working aspart of the drag reduction system. Nevertheless, such a system would notbe practical for a ground vehicle.

U.S. Pat. No. 5,222,698 is directed at a system that uses turbulencemonitors to determine whether the boundary layer is laminar orturbulent. In the event turbulent flow is detected, suction from anexternal source can be applied to the boundary layer until laminarboundary layer flow is reestablished. The system is designed for highlystreamlined aircraft structures—engine nacelles. Unlike the golf ballexample cited above, boundary layer separation can be avoided with thesehighly streamlined structures. In the case of highly streamlinedsurfaces traveling at aircraft speeds, drag is minimized by keepingboundary layer flow laminar and thus reducing the skin friction drag.

U.S. Pat. No. 5,348,256 and U.S. Pat. No. 5,417,391 also describesystems that utilize active control of the boundary layer throughsuction. In particular, U.S. Pat. No. 5,348,256 suggests reducing noiseon supersonic aircraft by allowing operation at higher angles of attackand reduced engine power levels. U.S. Pat. No. 5,417,391 discloses aseries of vortex chambers acting in concert with a suction source tocontrol the boundary layer flowing over an aircraft wing for thepurposes of increasing the wing's lift/drag ratio. Like the otherdisclosures discussed above, these systems are designed for aircraftapplications and do not identify the source of the suction.

U.S. Pat. No. 5,407,245 addresses the reduction of drag on a groundvehicle. Specifically, this patent disclosure identifies the “pressureresistance”—i.e. the creation of the low pressure wake behind avehicle—as the chief contributor to vehicle drag. Indeed, it isdisclosed that the form drag can be more than six times the skinfriction drag. The low pressure wake is reduced utilizing a blower atthe vehicle's rear to inject a high speed jet of air, which is estimatedto require a speed of 50 m/s (approximately 110 miles/hour)—at thelocation where the boundary layer would otherwise separate from thevehicle. The high speed jet of air adds energy to the boundary layer andprevents boundary layer separation and the attendant increase in formdrag that boundary layer separation produces.

In addition, by virtue of the vehicle's shape, the orientation of thevarious slots used to inject the high speed air, and the “Coandaeffect,” a portion of the high energy air is also injected into the areaof low pressure immediately behind the vehicle. U.S. Pat. No. 5,417,391refers to this area as the “eddying zone,” and the resulting injectionof high energy air into the eddying zone coupled with the suction of airfrom the eddying zone further reduces vehicle drag. The design calls fortwo blowers to provide both the injection of high speed air to preventboundary layer separation and the suction to pull air through theeddying zone. This citation recognizes that any practical system mustsave more energy in the form of drag reduction than it requires inenergy to power the blowers.

U.S. Pat. No. 5,908,217 uses another design to prevent boundary layerseparation at the rear of the vehicle. In particular, the design uses a“source of compressed air” and control valves to regulate the injectionof high speed air in the boundary layer at the vehicle's rear. A controlsystem is utilized to direct the flow of air through several differentplenums. By directing air through different plenums not only can thedrag on the vehicle be reduced, but it is possible to control thepitching and yawing moments on the car as well as providing improvedstability. Similarly, U.S. Pat. No. 6,068,328 proposes a boundary layercontrol system utilizing a series of “external perforation arrays andsuction sources controlled by a digital signal processor.” The designutilizes a series of turbulence detectors to direct suction to thoselocations at risk of boundary layer separation.

Moreover, U.S. Pat. No. 7,810,867 suggests a vehicle wrapping productdesigned to fit over a vehicle's existing structure. The wrappingproduct is designed to induce turbulence in the boundary layer flowingover the vehicle. The “tripping” of the boundary layer from laminar flowto turbulent flow is intended to delay the onset of boundary layerseparation and reduce the form drag. Essentially, the vehicle wrappingproduct performs a similar role to the dimples on a golf ball.

SUMMARY OF THE DISCLOSURE

Disclosed herein is a device for reducing aerodynamic drag of a groundvehicle, which includes a tubing having an intake port, an exit port,and a plurality of venturis, as well as a compressed air source beingfluidly connected to the intake port, wherein the plurality of venturisis disposed between the intake port and the exit port, and wherein eachventuri from among the plurality of venturis has a throat and a venturiopening.

Further, a method of reducing aerodynamic drag of a ground vehicle isdisclosed, which comprises providing a tubing on at least one of a roofand a side wall of the ground vehicle, the tubing having an intake port,an exit port, and a plurality of venturis disposed between the intakeport and the exit port; fluidly connecting a compressed air source tothe intake port; and, removing a portion of a fluid from a boundarylayer of the ground vehicle by suction through a venturi opening.

The present disclosure utilizes a series of venturi tubes to createsuction at the vehicle surface. The system is typically optimized aroundreducing form drag from tractor trailer trucks at highway speeds, but isapplicable to other vehicles and/or to other speeds. As drag varies withspeed squared, the system will produce the largest drag reduction athigher speeds. By optimizing the design around use in a certain speedrange, the device herein may be operated without any turbulencedetectors or complicated control system.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will now be described with reference to the drawingswherein:

FIG. 1 depicts the formation of a boundary layer 4 that is formed by thefreestream of air over stationary wall 2.

FIG. 2 depicts an attached boundary layer flow 7 around an airfoil 6.

FIG. 3 depicts a separated boundary layer flow 10 and the low pressurewake 11 behind a car 8.

FIG. 4 depicts a separated boundary layer flow 10 from an airfoil 6 at ahigh angle of attack 12.

FIG. 5 depicts a separated boundary layer flow 13 from a smooth golfball.

FIG. 6 depicts the delayed separation of boundary layer 13 from adimpled golf ball 15.

FIG. 7 depicts the current flow of air 10 over a tractor 16 and trailer17.

FIG. 8 depicts the flow 10 over a tractor trailer with aerodynamicfairing 18.

FIG. 9 depicts a boundary layer control system, which includes aplurality of tubings 20 attached to the fairing 18 of tractor 16 and tothe side walls and the roof of trailer 17.

FIG. 10 shows a top view of FIG. 9.

FIG. 11 shows a tubing 20 with two venturis 23 and 27 for creation ofsuction.

FIG. 12 shows an enlarged view of the rectangular area in FIG. 10 todepict venturis exhausting to vortex generators 21 at the trailing edge19 of trailer 17.

FIG. 13 depicts an exit port of a tubing 20 being formed as a diffuser31.

FIG. 14 depicts a schematic of ECU 32 control of blower 35 through dutysolenoid 33.

FIG. 15 depicts a drain system and a schematic layout of tubings on caband trailer.

FIG. 16 depicts an alternate layout with discharge valve 42 to allowpurging.

DESCRIPTION OF THE BEST AND VARIOUS EMBODIMENTS

The foregoing and other objects, aspects, and advantages will be betterunderstood from the following detailed description of the best andvarious embodiments. Throughout the various views and illustrativeembodiments of the present disclosure, like reference numbers are usedto designate like elements.

In a preferred embodiment, the boundary layer control system forreducing form drag is installed on a tractor trailer, which is a largebluff body otherwise producing large amounts of form drag. However, theinstant boundary layer control system can be installed and usedefficiently in a variety of ground vehicle applications as long as theincreased vehicle efficiency from the reduction in form drag exceeds theenergy required to power the source of compressed air. To achieve thisefficiency, a compressed air source is controlled by an engine controlunit (ECU) to inject compressed air into the intake port only when theground vehicle is traveling at a predetermined speed or faster.

As a source of compressed air an engine auxiliary, such as the engine'ssupercharger, turbocharger, or compressor, or a blower dedicated to thetask supplies a flow of high velocity air to a network of tubings havingventuris. In a preferred embodiment, a supercharger is used as thecompressed air source. Modern superchargers are typically driven by aclutched pulley which is activated by a duty cycle solenoid. Theengine's electronic control unit (ECU) monitors a variety of engineperformance characteristics and determines if the supercharger's outputis required. If the output is not required, the supercharger's clutchremains disengaged and the supercharger does not act as a parasitic dragon the engine output. If the supercharger's output is required, thesupercharger's clutched pulley is engaged via the duty cycle solenoid.The engine then powers the supercharger and the supercharger's output ofcompressed air is directed to the engine. A design of this typeminimizes the parasitic load of the supercharger on the engine.Integration of the proposed drag reduction system with a superchargercontrolled in this fashion will not introduce additional parasitic loadswhen the system is not in operation at reduced speed. At a speed whenthe drag reduction system is engaged, the ECU directs a portion of thesupercharger output to the drag reduction system.

In another preferred embodiment, a turbocharger is used as a source ofcompressed air. Utilization of a turbocharger for the supply of highvelocity air to the network of venturis does not introduce additionalparasitic loads when the vehicle is not in service. Turbochargers aredriven by the engine exhaust and are not driven directly from the engineas superchargers are. As with using a supercharger for the source ofhigh speed air to the network of venturis, the use of a turbocharger assource of high speed air is controlled via the ECU.

In yet another preferred embodiment, a dedicated blower is used as asource of compressed air. For the purpose of minimizing the impact onthe ECU a blower dedicated for use with the drag reduction system can beutilized. The dedicated drag reduction system blower can be controlledwith the same type of duty solenoid controlled clutched pulley utilizedwith most modern superchargers. This eliminates any parasitic drag onthe engine from the drag reduction system's blower when the system isnot in operation. When the vehicle speed increases sufficiently towarrant the start of the active drag reduction system the output of thededicated drag reduction system blower does not impact the amount of airbeing directed to the engine. By separating the source of air for thedrag reduction system from the source of engine combustion air the ECUlogic is simplified.

Regardless of the source of compressed air, the compressed air isinjected into a network of tubings comprising venturis. In a preferredembodiment, a plurality of venturis is connected in series within atubing. In the context of this disclosure, a plurality means two ormore. Typically, the tubing is prepared by taking a cylindrical tubingand providing swaged sections to produce the contour depicted in FIG.11. At the locations of reduced diameters, openings within the tubingwall are provided to thereby create the venturis. Preferably, thematerial for the tubing is chosen to be comparatively light weight butsturdy enough to retain its shape under the operating conditions of theground vehicle. The tubings can be made from any number of materialsincluding metals such as aluminum or stainless steel. However, thetubing material is not particularly limited and other materials suitablefor the purpose may be employed.

Preferably, a plurality of tubings is provided on at least one side walland/or the roof of the ground vehicle. Each of the tubings has aplurality of venturis, but the tubings may have a different number ofventuris. For example, for installation on a tractor trailer shortertubings with fewer venturis may be installed on the cab than on thetrailer. In a preferred embodiment, the plurality of tubings isconnected in parallel to the compressed air source. With preference, thetubing or tubings are aligned in a longitudinal direction of the groundvehicle.

The venturis create suction without using any moving parts. Suction iscreated by reducing the cross-sectional area of the tubing the air flowsthrough, i.e., by forming a venturi throat within the tubing. As thecross-sectional flow area is reduced, the velocity of the air increases(sub-sonic flow). As the air's velocity increases its static pressuredecreases. The reduction in static pressure then serves as a source ofsuction for the drag reduction system. The resulting combined flow ofcompressed air and low velocity air pulled in from the boundary layer isexhausted from the exit port in a high velocity jet to further preventboundary layer separation downstream of the drag reduction system'sventuris. In a preferred embodiment, the exit port of the tubing isformed as a diffuser as shown in FIG. 13. In another preferredembodiment, the opposite end of the tubing's intake port is used as theexit port and the exit port has the same diameter as the intake port.

Preferably, the tubings and the throats have circular cross-sections,but other cross-sections may also be selected. For example, thecross-sections may have a rectangular, square, triangular, elliptical,oval or semicircle shape, but other shapes are also possible. Further,the shape of the cross-section of the tubing may be the same ordifferent from the shape of the throat. Typically, the venturi throathas a cross-section of from 0.5 cm to 10 cm. Also typically, the venturiopening has a cross-section of from 0.5 cm to 10 cm. With preference,the distance between two adjacent venturis from among the plurality ofventuris is of from 10 cm to 200 cm. Also with preference, the distancebetween two adjacent tubings from among the plurality of tubings is offrom 10 cm to 120 cm.

In a preferred embodiment, the plurality of venturis within one tubinghave the same throat cross-section and the same venturi opening diameterbecause such an arrangement simplifies the manufacture of the tubings.However, in another preferred embodiment, throat cross-sections and/orventuri openings may be different within one tubing or between differenttubings. In a particularly preferred embodiment, a venturi from amongthe plurality of venturis that is disposed closer to the intake port hasat least one of a larger throat cross-section and a larger venturiopening diameter than a venturi being disposed closer to the exit port.

Optionally, the high velocity jet produced by the active boundary layercontrol system is directed from the exit port (with or without adiffuser) to passive boundary layer control devices or vortexgenerators. The vortex generators can take a variety of shapes, but aretypically V-shaped with the tip pointing toward the trailing edge of theground vehicle. Regardless of the shape the vortex generators take, theyadd fresh momentum to the boundary layer and further prevent separation.With preference, one or more vortex generators is disposed adjacent tothe exit port of a tubing. In the context of this disclosure, adjacentmeans that the distance between the exit port and the vortexgenerator(s) is chosen such that at least a portion of the jet exhaustedfrom the exit port flows over a vortex generator when the system is inoperation. Thus, there may be a considerable distance between the exitport and the adjacent vortex generator. In a preferred embodiment, thedistance between the vortex generator and the exit port is of from 10 cmto 500 cm.

Preferably, the vortex generators are placed as close to the vehicle'strailing edge as possible, but should not be placed downstream of theseparation point. As the overall drag reduction system is preferablydesigned and optimized around highway speeds, the separation point ofinterest should remain in a limited area on the vehicles surface. Inorder to minimize the drag the vortex generators produce when the activeboundary layer control system is not active—in other words at lowervehicle speeds—the height of the vortex generators preferably remainsbelow the local thickness of the boundary layer.

In a typical embodiment, the boundary layer control system is built intothe sidewalls and/or roof of the ground vehicle during the production ofthe vehicle. However, in another typical embodiment, the boundary layercontrol system is mounted onto an existing ground vehicle.

In order to accommodate the presence of water and debris the dragreduction system may be equipped with solenoid type control valves(on/off). The valves ensure that the system is free of debris and waterat start-up and remains clean during operation. Drain valves are used toprevent the accumulation of water in the system when the system is notin service. Before air can be directed to the active portion of theboundary layer control system, the drain valve must be confirmed open. Acontact switch is provided for this purpose. Once the source ofcompression is engaged the solenoid operated drain valves close. Uponshutdown of the active portion of the boundary layer control system thesolenoid valve returns the drain valve to the open position. Finally, ananti-icing heating element 43 may be provided as needed to prevent theformation of ice in the system. However, depending on the ambientconditions the heat of compression alone may be sufficient to preventthe formation of ice.

In a preferred embodiment, the ground vehicle is selected from the groupconsisting of a tractor trailer truck, a train, a trailer, a deliverytruck, a minivan, a station wagon, and a sports utility vehicle. In aparticularly preferred embodiment, the ground vehicle is a tractortrailer truck.

FIG. 7 shows the flow of air 1 around the cab 16 and the trailer 17 of aconventional tractor trailer truck or “18-wheeler.” The sharpdiscontinuity between the cab 16 and the trailer 17 causes a largeamount of flow separation 10 which then contributes to a large lowpressure wake 11 behind the truck. The pressure inside the wake 11 islower than the pressure at the front of the truck. This pressuredifferential causes a net force in the opposite direction the vehicle ismoving. Additional fuel must be fed to the truck's engine to counteractthis drag force, which is, of course, form drag.

An attempt at reducing the vehicle's drag and increasing the fuelefficiency can be seen in FIG. 8, which shows a cab 16 and trailer 17 ofa tractor trailer truck. The cab 16 has been fitted with a fairing 18.The fairing 18 eliminates the sharp discontinuity between the cab 16 andthe trailer 17. As a result, the air flowing over the cab 16 is able toflow over the fairing 18 without separating. By avoiding separation theform drag is reduced at the transition from the cab 16 to the trailer17. The boundary layer separates at separation point 13.

However, a significant source of form drag remains and that is the flowof air over the trailer's trailing edge 19. As with the flow of airbetween the cab 16 and the trailer 17 in FIG. 7, the air flowing overthe trailing edge 19 in FIG. 8 is unable to turn inward because of thetrailer's shape. Because the air cannot turn inward it separates fromthe vehicle's trailing edge 19 at the separation point 13 and a lowpressure wake 11 is produced. In the same way that the fairing 18 wasadded to smooth out the transition from the cab 16 to the trailer 17,aerodynamic streamlining features could be added to the trailer 17.

Nevertheless, there are practical problems associated with utilizingstreamlining at the trailer's trailing edge 19. For example, there arelimits on the length of tractor trailers and any sort of effectivestreamlining that could be provided while still ensuring sufficienttransport capacity for goods. In addition, depending on the aerodynamicstreamlining features used there is the very real prospect that thesefeatures would interfere with the operation of the trailer doors, inparticular when approaching a loading dock or during on- and off-loadingof the truck. Moreover, streamlining features that impair the truckdriver's view should be avoided. The boundary layer control systemdisclosed herein provides for a reduction in form drag without impedingthe practicality of the ground vehicle.

Consequently, the instant disclosure achieves a reduction of the formdrag from ground vehicles by delaying the drag inducing flow separationoccurring at a vehicle trailing edge or any sharp shape discontinuity.It is the separation of flow that then produces the large low pressurewake, which contributes such a large component to the overall vehicle'saerodynamic drag. Similarly to the dimpled golf ball of FIG. 6, areduction in aerodynamic drag is achieved by delaying the separation ofthe boundary layer.

A preferred embodiment of a device for reducing aerodynamic drag of aground vehicle can be seen in FIG. 9. As before there is a freestream ofair 1, which flows over a truck comprised of cab 16 and trailer 17. Thecab has been fitted with a fairing 18 to reduce the flow separation thatmay occur as the air transitions between the cab 16 and the trailer 17.However, active and passive elements of a drag reduction system havebeen added. The active element uses the introduction of energy fromanother source to reduce form drag. A passive element may be coupled tothe active element to increase efficiency of drag reduction. The passiveelement utilizes the energy that is already in the flowing stream ofair. In a preferred embodiment, the active element is a network ofconnected tubings 20, each tubing having multiple venturis. The passiveelement is an array of vortex generators 21 disposed upstream of whatwould otherwise be the separation point for the air flowing over thetrailer. Typically, the separation point 13 as shown in FIG. 8 for atrailer truck without a boundary layer control system is formed at adifferent location relative to the trailer than the separation point ofa truck with a boundary layer control system. In a preferred embodiment,one vortex generator 21 is provided per tubing 20. In another preferredembodiment, two or three vortex generators are provided per tubing 20.In yet another preferred embodiment, the passive aerodynamic boundarylayer control devices are only provided at the trailing edge 19 of thetrailer 17.

As shown in FIG. 10, the network of venturis are located toward theleading edge of the trailer and integrated into the fairing 18 thatassists the air in transitioning from flowing over the cab 16 to flowingover the trailer 17. FIG. 10 shows a top view of the trailer truck ofFIG. 9. The boundary layer control system does not require the use of afairing 18, but the benefits of the fairing 18 enhance the disclosedboundary layer control system. The use of a fairing 18 also minimizesthe amount of energy needed for providing compression to the network ofventuris.

High pressure air is injected into the tubings 20 having a plurality ofventuris. An elevation view of a representative section of a pluralityof venturis in a tubing is depicted in FIG. 11. As can be seen in FIG.11, each individual venturi 23, 27 contains an area of minimumcross-sectional area, the throat 22, 28. For the pressure ranges ofinterest in the disclosed device, the air flows at sub-sonic speed. Asthe cross sectional area of the tubing 20 contracts the speed of the air24 flowing in the tubing increases. The airspeed continues to increaseuntil the air flows within the throat 22. The energy of the air 24flowing through the network of tubings must be conserved. Theconservation of energy relationship for a flow of this type is mosteasily expressed using Bernoulli's law. This law states, among otherthings, that as the velocity of a fluid is increased its static pressurewill drop. As a result, while the air's velocity is highest at theventuri throat 22, the air's static pressure is lowest.

The low pressure created at the throat acts as a source of suction anddraws in low energy air 25 from the boundary layer through the venturiopening. This low energy air is the air that would otherwise separate asflow 10 at separation point 13 in FIG. 8. By reducing or avoidingseparation, form drag is reduced. The air 26 downstream of the throat 22is now comprised of low energy air from the boundary layer 25, as wellas the air originally injected into the tubings 20. This air 26 flows tothe next downstream venturi 27, where the process seen at the upstreamventuri 23 is repeated. Namely, air 26 flows into the downstream venturithroat 28 where the air's velocity increases and its pressure decreases.The reduced pressure at the throat 28 acts as a source of suction anddraws in low energy air 25 from the boundary layer air 10 flowing overthe trailer 17 and prevents or minimizes separation.

In a preferred embodiment, the instant method of reducing aerodynamicdrag of a ground vehicle includes providing a vortex generator adjacentto the exit port. In another preferred embodiment, the method includesdetermining a minimum vehicle speed at which injecting compressed airfrom the compressed air source into the intake port saves more energy byreducing the aerodynamic drag of the ground vehicle than is expended bythe compressed air source injecting the compressed air into the intakeport; and, injecting the compressed air only when the ground vehicle istravelling at least at the minimum vehicle speed. With preference, theminimum vehicle speed is from 40 mph to 80 mph.

In a preferred embodiment, the network of tubings containing theventuris does not run the entire length of the trailer 17. As depictedin FIG. 10, the tubings extend over approximately 80% of the length ofthe trailer. However, other lengths of tubings may be provided. In apreferred embodiment, the lengths of the tubings extend between 50% and95% of the length of the trailer. In another preferred embodiment, thenetwork of tubings 20 runs long enough to prevent large scale separationof flow and the attendant production of form drag. Further reduction ofboundary layer separation is accomplished by exhausting the air from thenetwork of tubings 20 in a concentrated jet 29 from the exit port of thetubings, as depicted in FIG. 12. The concentrated jet 29 is directedfrom the exit port to vortex generators 21. The flow of the concentratedjet 29, coupled with the action of the vortex generators 21, increasesthe energy of the air 30 flowing over the trailing edge 19. The combinedeffect of the jet and the vortex generators is to reduce trailing edgeseparation and with it aerodynamic drag on the vehicle by a reduction inform drag. In analogous manner to the dimples 15 on the golf ball 14seen in FIG. 6, the active and the optional passive control elementsprevent or reduce boundary layer separation.

In another preferred embodiment, the boundary control system employsboth active and passive boundary layer control elements. By virtue ofusing the same stream of air as both a source of suction for theventuris and the concentrated jet 29, the maximum benefit accruing fromthe energy necessary to provide the compressed air 24 is obtained.

In the preferred embodiment depicted in FIG. 13, a diffuser 31 isdisposed at the exit port of the tubing. Thus, air exhausting from theexit port of tubing 20 is streaming through a diffuser 31 where the airloses momentum before flowing to the vortex generator(s). All of theexit ports or only a subset thereof may be provided with diffusers. Thediffusers may be made from the same material that the tubing may bemade, and the diffusers may be made simultaneously with the tubings orsubsequently attached. With preference, the boundary layer controlsystem is optimized to achieve the largest reduction in form drag whenthe ground vehicle is traveling at highway speeds. However, thedisclosure is not limited to highway speeds. Rather, the system can beadapted to various speeds. For example, if the form drag reductiondevice is mounted on a train, the device is optimized for speed rangeshigher than highway speed. Optimization is preferably accomplished byvarying one or more of the following: the flow rate of the injected air24, the diameter of the tubing 20, the diameter of the venturi openings,the cross-section of the throats, the distance between and the number ofventuris within a tubing, the length of the tubing, the distance betweentubings attached to a side wall or the roof of the ground vehicle, ifpresent, the distance between the vortex generator(s) and the exit portof the tubings, the size and shape of the vortex generators, and thenumber of vortex generators per tubing.

In a preferred embodiment, no compressed air is injected into thetubings for those speeds where the reduction in form drag is notsufficiently large to justify the energy consumption in the activeboundary layer control system. As the aerodynamic drag increasesexponentially with speed, the majority of form drag reduction benefitsare achieved at higher vehicle speeds. Optimizing the system to operatein a specific range of speeds also greatly minimizes the number ofcomponents needed to achieve form drag reduction. In a preferredembodiment, the system is installed without sensors for monitoringturbulence or pressure over any part of the vehicle. However, the use ofsuch sensors is not precluded. In addition, because the flow patternsremain fairly constant at typical vehicle speeds of ground vehicles, thenetwork of tubings 20 can be located at fixed vehicle locations andstill provide the maximum drag reduction benefit. Thus, it is possibleto manufacture large numbers of tubings with predetermined dimension toreduce the cost per tubing while still providing energy savings todifferent kinds of ground vehicles. However, for optimal form dragreduction for a specific ground vehicle, the above-listed parameters areoptimized to provide a tailored boundary layer control system for thatspecific ground vehicle.

The source of high pressure air 24 that flows through the network oftubings is provided by a work consuming device of some type—blower,turbocharger, supercharger or compressor. Large commercial trucks arealmost always fitted with a supercharger or turbocharger. A portion ofthe air discharging from these devices can be injected into the networkof tubings 20. In a preferred embodiment, the device for producing airflow 24 is a turbocharger. Turbochargers are powered by engine exhaustgases and not directly by the engine itself as superchargers are.Moreover, turbochargers are fitted with a waste-gate or bypass valvethat directs some of the useful turbocharger output away from the engineintake to avoid overstressing the engine. In circumstances where thewaste-gate or bypass valve would otherwise be open, this excessturbocharger output is injected into the tubings 20 and the additionalenergy required to operate the active boundary layer control system isminimized.

Utilization of an existing engine auxiliary like a supercharger orturbocharger for the source of high pressure air 24 typically requiresmodification of the engine's electronic control unit (ECU). The ECUmonitors a variety of engine and vehicle variables and directs thecorrect amounts of air and fuel to the engine. Thus, the ECU mustaccount for compressed air injected into the active boundary layercontrol system instead of the engine. In addition, and in spite of thefact that the additional energy consumption of the compressed air sourceis less than the energy savings resulting from the reduction in formdrag, the capacity of the supercharger or turbocharger may need to beincreased to accommodate the air flow of the active boundary layersystem. An active boundary layer control system that integrates existingengine auxiliaries—turbochargers, superchargers—into its design can bemore easily integrated into existing vehicles than an active boundarylayer control system that utilizes a dedicated source of high pressureair. However, a source of high pressure air 24 provided by a dedicatedblower or compressor simplifies modifications to the ECU because thesource of combustion air for the engine and high pressure air for theboundary layer control system are independent.

Regardless of the source of high pressure air the system is only engagedat vehicle speeds sufficiently high to warrant the parasitic energyconsumption of driving the source of high pressure air. For componentspowered by the engine, for example a supercharger or a blower whoseoutput is dedicated to the active boundary layer control system,engaging/disengaging the active boundary layer control system is easilyintegrated into either existing or new engines by the use of a dutysolenoid and a clutched pulley, see FIG. 14. In particular, FIG. 14shows an engine ECU 32, duty solenoid 33, clutched pulley 34, and asource of high pressure air 35, which in this case is a blower. When theECU 32 determines that the vehicle is traveling sufficiently fast toengage the active boundary layer control system, the ECU sends a signalto the duty solenoid 33. The duty solenoid then engages the clutchedpulley and the engine is able to power the source of high pressure airfor the active boundary layer control system.

There are some mechanical aspects of the system that are required toaccommodate exposure to the atmosphere and the system's physical layout.Because the system can be exposed to rain and the elements, it isadvantageous to integrate drains into the system design to keep waterfrom damaging the source of high pressure air 35 (blower, compressor,supercharger, turbocharger). In a preferred embodiment, much of theactive boundary layer control system is located at the top of the groundvehicle, and a low point drain is incorporated into the system, asdepicted in FIG. 15. Specifically, FIG. 15 shows a drain layout thatincludes a solenoid operated drain valve 36 and check valve 37. Apermissive for starting the blower, supercharger or other source of highpressure air 35 for the active boundary layer control system is for thesolenoid operated drain valve to be open. The open/closed status of thesolenoid valve 36 is monitored with a contact switch 38. Preferably, thesolenoid operated drain valve remains open while the active boundarylayer system is not in operation to prevent accumulation of water in thesystem. Once the blower, supercharger, or other source of high pressureair 35 for the active boundary layer control system is up to speed andproducing pressure, the solenoid operated drain valve 36 closes tomaximize the suction created at the venturis.

In a preferred embodiment, the active portion of the boundary layercontrol system is provided on large commercial vehicles like trucks.Typically, the system is installed such that it is possible to separatethe active elements that act on the cab and the trailer. FIG. 15 showstwo active boundary layer control systems 39 and 40, which act on thecab and on the trailer, respectively. Also typically, the activeboundary layer control systems 39 and 40 are connected to the source ofcompression via expansion joints 41. The expansion joints 41 allow thesystem to withstand thermal expansion and any relative motion betweenthe cab 16 and the trailer 17. Further, the expansion joint 41 alsoallows for a relatively quick disconnect between the cab 16 and thetrailer 17, when desired.

In yet another preferred embodiment, discharge valve 42 can beintegrated into the system as well, see FIG. 16. This valve—or valves ifmore than one valve is provided—remains closed as the source ofcompression begins to generate pressure in the active boundary layercontrol system. By remaining shut, air is forced through the venturis topartially or completely prevent the accumulation of dirt or debris inthe system. After a few seconds in the closed position the dischargevalve 42 opens and the venturis are able to create suction. Heatingelement 43 as shown in FIG. 15 is controlled by the ECU and preventsformation of ice or melts ice that has been formed in the tubings whenthe vehicle has not been used for a while.

The embodiments described hereinabove are further intended to explainbest modes known of practicing it and to enable others skilled in theart to utilize the disclosure in such, or other, embodiments and withthe various modifications required by the particular applications oruses. Accordingly, the description is not intended to limit it to theform disclosed herein. Also, it is intended that the appended claims beconstrued to include alternative embodiments.

The foregoing description of the disclosure illustrates and describesthe present disclosure. Additionally, the disclosure shows and describesonly the preferred embodiments but, as mentioned above, it is to beunderstood that the disclosure is capable of use in various othercombinations, modifications, and environments and is capable of changesor modifications within the scope of the concept as expressed herein,commensurate with the above teachings and/or the skill or knowledge ofthe relevant art.

The term “comprising” (and its grammatical variations) as used herein isused in the inclusive sense of “having” or “including” and not in theexclusive sense of “consisting only of.” The terms “a” and “the” as usedherein are understood to encompass the plural as well as the singular.

All publications, patents and patent applications cited in thisspecification are herein incorporated by reference, and for any and allpurpose, as if each individual publication, patent or patent applicationwere specifically and individually indicated to be incorporated byreference. In the case of inconsistencies, the present disclosure willprevail.

What is claimed is:
 1. A device for reducing aerodynamic drag of a ground vehicle, the device comprising: a tubing having an intake port, an exit port, and a plurality of venturis; and, a compressed air source being fluidly connected to the intake port; wherein the plurality of venturis is disposed between the intake port and the exit port; and, wherein each venturi from among the plurality of venturis has a throat and a venturi opening on an outer periphery of the venturi throat to remove a portion of a fluid from a boundary layer of the ground vehicle.
 2. The device according to claim 1, wherein the compressed air source is selected from the group consisting of an engine auxiliary and a dedicated blower.
 3. The device according to claim 1, wherein the compressed air source is controlled by an engine control unit to inject compressed air into the intake port only when the ground vehicle is travelling at a predetermined speed or faster.
 4. The device according to claim 1, wherein the plurality of venturis is connected in series.
 5. The device according to claim 1, wherein a plurality of tubings is provided on at least one of a side wall and a roof of the ground vehicle; and, wherein the plurality of tubings is connected in parallel to the compressed air source.
 6. The device according to claim 5, wherein a distance between two adjacent tubings from among the plurality of tubings is of from 10 cm to 120 cm.
 7. The device according to claim 1, further comprising a diffuser being disposed at the exit port.
 8. The device according to claim 1, further comprising a vortex generator being disposed adjacent to the exit port.
 9. The device according to claim 8, wherein the vortex generator is disposed at a trailing edge of the ground vehicle.
 10. The device according to claim 8, wherein a distance between the vortex generator and the exit port is of from 10 cm to 500 cm.
 11. The device according to claim 1, wherein the tubing further comprises at least one of an internal heating element and a drain valve.
 12. The device according to claim 1, wherein the plurality of venturis have a same throat cross-section and a same venturi opening diameter.
 13. The device according to claim 1, wherein the tubing is aligned in a longitudinal direction of the ground vehicle.
 14. The device according to claim 1, wherein the throat has a cross-section of from 0.5 cm to 10 cm.
 15. The device according to claim 1, wherein the venturi opening has a cross-section of from 0.5 cm to 10 cm.
 16. The device according to claim 1, wherein a distance between two adjacent venturis from among the plurality of venturis is of from 10 cm to 200 cm.
 17. The device according to claim 1, wherein the ground vehicle is a tractor trailer truck.
 18. A method of reducing aerodynamic drag of a ground vehicle comprising: providing a tubing on at least one of a roof and a side wall of the ground vehicle, the tubing having an intake port, an exit port, and a plurality of venturis disposed between the intake port and the exit port; fluidly connecting a compressed air source to the intake port; and, removing a portion of a fluid from a boundary layer of the ground vehicle by suction through a venturi opening located on an outer periphery of each throat venturi.
 19. The method of claim 18, further comprising: providing a vortex generator adjacent to the exit port.
 20. The method according to claim 18, further comprising: determining a minimum vehicle speed at which injecting compressed air from the compressed air source into the intake port saves more energy by reducing the aerodynamic drag of the ground vehicle than is expended by the compressed air source injecting the compressed air into the intake port; and, injecting the compressed air only when the ground vehicle is travelling at least at the minimum vehicle speed.
 21. The method according to claim 20, wherein the minimum vehicle speed is of from 40 mph to 80 mph. 